What you'll learn
Your body must keep the amount of water and the level of waste substances in your blood within narrow limits, and the kidneys are central to this control. For AQA GCSE Biology you need to understand why water and nitrogen balance matter, how the body loses water and nitrogen, how the kidneys filter the blood and reabsorb what is needed, and how the hormone ADH controls water balance. This guide covers the principles of homeostasis applied to water and nitrogen, the formation of urea, filtration and selective reabsorption, and the negative feedback control of water. By the end you should be able to explain how the kidneys maintain balance and how ADH adjusts water levels.
Key terms and definitions
Homeostasis — The regulation of internal conditions to maintain a stable environment for cells.
Urea — A waste product made in the liver from excess amino acids, removed by the kidneys.
Deamination — The breakdown of excess amino acids in the liver, producing ammonia which is converted to urea.
Kidney — The organ that filters the blood, removing urea and adjusting water and ion levels.
Filtration — The process in the kidney where small molecules are forced out of the blood.
Selective reabsorption — Taking useful substances (such as glucose) back into the blood from the kidney.
ADH (anti-diuretic hormone) — A hormone that controls how much water is reabsorbed by the kidneys.
Negative feedback — A control mechanism that reverses a change to return a condition to normal.
Core concepts
Why water and nitrogen balance matter
The cells of the body work best in stable conditions. If the water content of the blood changes too much, water will move into or out of cells by osmosis, which can damage them. If waste nitrogen compounds build up, they are toxic. So the body must constantly control both the amount of water and the level of nitrogen waste in the blood — this is part of homeostasis.
How the body loses water
Water is lost from the body in several ways:
- Through the lungs when breathing out.
- Through the skin in sweat (which also loses ions and helps cool the body).
- Through the kidneys in urine.
The body cannot control the water lost through the lungs or in sweat, so the balance is adjusted by the kidneys, which control the water lost in urine.
Making urea (nitrogen balance)
The body cannot store excess protein or amino acids. When there are more amino acids than the body needs, they are broken down in the liver by a process called deamination. This produces ammonia, which is very toxic, so the liver immediately converts it into urea. Urea is less toxic and is carried in the blood to the kidneys, where it is removed in the urine.
How the kidneys work
The kidneys clean the blood in two main steps:
- Filtration — Blood is filtered at high pressure, so small molecules such as water, glucose, ions and urea are forced out of the blood, while large molecules such as proteins and blood cells stay behind.
- Selective reabsorption — Useful substances are taken back into the blood. All the glucose is reabsorbed, along with the water and ions the body needs. The urea and excess water and ions are not reabsorbed and leave the body as urine.
This means the kidneys remove waste while keeping the useful substances the body needs.
Controlling water balance with ADH
The amount of water reabsorbed by the kidneys is controlled by the hormone ADH, released by the pituitary gland. This is an example of negative feedback:
- If the blood is too concentrated (too little water), more ADH is released. This makes the kidneys reabsorb more water, so a small volume of concentrated urine is produced and the blood water level rises back to normal.
- If the blood is too dilute (too much water), less ADH is released. The kidneys reabsorb less water, so a large volume of dilute urine is produced and the blood water level falls back to normal.
In both cases the change is detected, a response reverses it, and the condition returns to normal — the definition of negative feedback.
Why water balance affects cells
The reason water balance matters so much comes down to osmosis. Water moves across cell membranes from a more dilute solution to a more concentrated one. If the blood becomes too dilute, water moves into the cells by osmosis, and they can swell and even burst. If the blood becomes too concentrated, water moves out of the cells, and they shrink and cannot work properly. By keeping the water content of the blood steady, the kidneys and ADH protect the body's cells from these damaging changes. This is why the control of water is a good example of homeostasis keeping conditions stable for cells.
Controlling ion (salt) balance
As well as water and urea, the kidneys control the level of mineral ions such as sodium in the blood. Ions are taken into the body in food, and if there are too many, the excess must be removed. During selective reabsorption, only the ions the body needs are reabsorbed into the blood; any excess ions are left in the urine and removed. This keeps the concentration of ions in the blood at the right level, which is important because ion concentration also affects the movement of water by osmosis. Ion balance and water balance are therefore closely linked.
Putting it together
The kidneys therefore do two jobs at once: they remove the toxic waste urea, and they adjust the water and ion content of the blood. ADH allows the water balance to be fine-tuned from moment to moment, depending on how much water a person has drunk, how much they have sweated, and what they have eaten.
Worked examples
Example 1: The origin of urea
Explain how urea is produced and removed. Excess amino acids are broken down in the liver by deamination, producing toxic ammonia, which the liver converts to urea. The urea is carried in the blood to the kidneys and removed in the urine.
Example 2: Filtration and reabsorption
State one substance that is filtered out and then completely reabsorbed, and one that is not reabsorbed. Glucose is filtered out and then completely reabsorbed into the blood. Urea is filtered out and not reabsorbed, so it leaves the body in the urine.
Example 3: ADH on a hot day
On a hot day a person sweats a lot and drinks little. Explain what happens to their ADH and urine. Their blood becomes too concentrated, so more ADH is released. The kidneys reabsorb more water, producing a small volume of concentrated urine, which conserves water and returns the blood to normal.
Example 4: Negative feedback
Explain why controlling water balance is an example of negative feedback. When the water level moves away from normal, it is detected and a response is triggered that reverses the change — for example, low water triggers more ADH and more reabsorption. Because the response opposes the original change, it is negative feedback.
Common mistakes and how to avoid them
A very common error is confusing high and low ADH. Remember: too little water → more ADH → more water reabsorbed → concentrated urine. Write it out as a chain to keep it straight.
Students often say the kidney "makes" urea. It does not — urea is made in the liver by deamination of excess amino acids. The kidney only removes it.
Another mistake is forgetting that proteins and blood cells are not filtered out. Filtration removes only small molecules; large molecules stay in the blood, which is why finding protein in urine can indicate a problem.
When describing reabsorption, be specific that all the glucose is reabsorbed, along with the water and ions the body needs, but urea is not.
Finally, do not confuse water lost in sweat with water controlled by the kidneys. The body cannot control sweat or breathing losses; it adjusts the balance through the urine.
Exam technique for "Maintaining water and nitrogen balance"
For nitrogen questions, give the full pathway: excess amino acids → deamination in the liver → ammonia → converted to urea → removed by kidneys in urine. Naming the liver and deamination is essential.
Kidney questions usually want the two steps: filtration of small molecules, then selective reabsorption of glucose, water and needed ions. State clearly what stays in the blood and what leaves as urine.
ADH questions are best answered as a negative-feedback chain: state whether the blood is too concentrated or too dilute, whether more or less ADH is released, whether more or less water is reabsorbed, and the effect on urine. Setting it out step by step earns every mark and avoids the common high/low mix-up.
Quick revision summary
- Homeostasis keeps water and nitrogen levels stable so cells are not damaged by osmosis or toxic waste.
- Water is lost through the lungs, skin (sweat) and kidneys; only the kidneys control the balance, via urine.
- Excess amino acids are deaminated in the liver to ammonia, converted to urea, and removed by the kidneys.
- Kidneys work by filtration (small molecules forced out) then selective reabsorption (glucose, needed water and ions taken back); urea leaves in urine.
- ADH controls water reabsorption by negative feedback: too little water → more ADH → more reabsorption → concentrated urine.
- Too much water → less ADH → less reabsorption → dilute urine.